1. Field of the Invention
This invention relates generally to an electromagnetic equipment comprising an electromagnetic coil such as transformers and dynamoelectric machines, and more particularly to such an electromagnetic equipment having improved corona resistance, dielectric strength and heat resistance.
2.Description of the Prior Art
The electromagnetic equipment of the above-described type includes induction machines such as dynamoelectric machines and transformers. Conventional induction machines comprise an electromagnetic coil formed by winding enameled wires serving as insulated wires. In the electromagnetic coil, generally, insulators deteriorate when exposed to corona discharge. In the prior art, however, the corona discharge cannot be controlled so as to reduce a quantity of electric charge discharged by the corona discharge, whereupon each insulator has a low deterioration resistance, namely, a low corona resistance. In the prior art electromagnetic coil, furthermore, an impulse voltage is unequally shared by coil portions. The unequal sharing of voltage tends to result in dielectric breakdown in a coil potion sharing an excessively high voltage. The prior art electromagnetic coil thus has a low dielectric strength. As a result, the electromagnetic coil has a difficulty in reducing a thickness of each insulator so that the size of the induction machine is reduced.
To solve the above-described problem, the prior art has provided two layers of polyester film 2 wound on a conductor 1 and mica foil 3 interposed between the polyester film layers, as shown in FIG. 13. Furthermore, the prior art has provided a polyester film 5 wound on a surface of a unit coil 4 having a plurality of turns and a mica tape 6 wound on a surface of the polyester film 5, as shown in FIG. 14. Since each prior art construction uses the mica foil 3 and the mica tape 6 respectively, they are superior in a resistance against electrical discharge, namely, in the dielectric strength. However, each construction is insufficient in the corona resistance since a quantity of electric charge discharged by the corona discharge cannot be restrained.